Last Updated : 12 Jul, 2025
JavaScript handler.preventExtensions() method in JavaScript is a trap for Object.preventExtensions() method. New properties cannot be added to an object when extensions are disabled. It returns a boolean value.
Syntax:
const p = new Proxy(target, {
preventExtensions: function(target) {
}
});
Parameters: This method accepts a single parameter target that holds the target object.
Return value: This method returns a boolean value.
Below examples illustrate the handler.preventExtensions() method in JavaScript:
Example 1: In this example, we will see the use of the handler.preventExtensions() method in JavaScript.
javascript
const p = new Proxy({}, {
preventExtensions: function (target) {
console.log('preventExtensions()');
console.log(Object.preventExtensions(target));
return true;
}
});
console.log(Object.preventExtensions(p));
let x = {
first: false
};
let y = {
preventExtensions(target) {
target.canEvolve = false;
Object.preventExtensions(target);
return true;
}
};
let proxy = new Proxy(x, y);
console.log(x.first);
Object.preventExtensions(proxy);
console.log(x.first);
Output:
true [object Object] false true
Example 2: In this example, we will see the use of the handler.preventExtensions() method in JavaScript.
javascript
const p = new Proxy({}, {
preventExtensions: function (target) {
console.log('preventExtensions()');
console.log(Object.preventExtensions(target));
return true;
}
});
console.log(Object.preventExtensions(p));
let x = {
first: false
};
let y = {
preventExtensions(target) {
target.canEvolve = false;
Object.preventExtensions(target);
return true;
}
};
let proxy = new Proxy(x, y);
console.log(x.first);
Object.preventExtensions(proxy);
console.log(x.first);
Output:
preventExtensions() [object Object] [object Object] false false
Supported Browsers: The browsers supported by handler.preventExtensions() method are listed below:
We have a complete list of Javascript Proxy/handler methods, to check those go through the Javascript Proxy/handler Reference article.
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